If you happened to glance through the personals on Hong Kong's Craigslist recently, you might have spotted one titled "50 Shades of something more interesting". A year ago, the headline would have made no sense. But now, thanks to a middle-aged English woman named E.L. James, everything is clear.

Having begun on the internet as an homage to Stephenie Meyer's Twilight series, the Fifty Shades trilogy ( Grey, Darker and Freed) narrates a sado-masochistic romance between a manipulative billionaire (the titular Christian Grey) and a winsome student (Anastasia Steele).

Its influence has spread rapidly - from well-heeled corners of metropolitan America to James' homeland, where its 5.4 million sales have made it the most popular novel in Britain ever. Now, with estimated sales of more than 40 million worldwide, Fifty Shades is a global phenomenon - and Asia is the latest continent to fall under James' spell. The Korea Times recently reported that Fifty Shades' first two instalments have sold more than 150,000 copies alone. Although its appeal in Hong Kong is restricted mostly to expat readers, editions in Chinese are being printed.

Nury Vittachi, journalist and author of The Feng Shui Detective, is not surprised by James' growing success in the region. "Asians could be said to have invented erotic writing. For most of recorded history, the best-known erotic book was The Kama Sutra. Japan's The Tale of Genji has been described as the first erotic novel. Others have ascribed that title to China's more readable Jin Ping Mei (The Plum in the Golden Vase)."

The sheer extent of James' popularity (she is rumoured to earn US$1.5 million a week) is now her strongest selling point. "You have to read it to be part of a cultural conversation," says Jonathan Beckman, an editor at Literary Review who organises the annual Bad Sex Awards. "It's not about reading for enjoyment. It's like reading the news."

But how do we explain the popular appeal of a book whose sexual content would normally consign it to a niche readership? Is its success a good thing - for women, sexuality, and literature?

Whatever the question, James divides opinion: not so much grey area as 50 shades of black and white. "You hear such different responses," Beckman says. "Some people praise the fact that they can identify with a story. Others complain that it takes 70 pages before you get to the juicy bits."

For Freya North, best-selling author of romances such as Rumours, Fifty Shades' "juicy bits" are an erotic anti-climax: "Readers are always happy to come across fruity passages. What's different with James is the sexual scenes outweigh the story - sex for sex's sake, rather than crafted erotica like Anais Nin."

North is in a minority. Although some complain that the plot gets in the way of the "juicy bits", many more admit to skimming the raunch to follow the main event: will Christian and Anastasia tie the knot, figuratively as well as literally? For Claire Siemaszkiewicz, founder of erotic romance publisher Total-E-Bound: "Women are titillated by great stories, putting themselves into the position of the heroine and enjoying an imagined suitor who satisfies their every need." Nikki Gemmell, author of 2003's erotic sensation The Bride Stripped Bare, agrees: " Fifty Shades is a fairy tale for grown women. Many love a fast, visceral read that gets you excited."

All this talk of fairy tales is too euphemistic for British actor Neil Pearson, a bibliophile and collector of erotic literature. "There's a distinction to be drawn between erotica and pornography. Fifty Shades is porn - not extreme porn, but it's porn. Nothing wrong with that, but to acknowledge it publically would be to acknowledge that its sole reason to exist is to make its readers come. And no-one connected with the book is going to be saying that anytime soon."

Pearson is unsure where Fifty Shades fits into a tradition of erotic literature: for example, L'Histoire d'O by Pauline Réage. " Fifty Shades readers would mostly be horrified by L'Histoire d'O, a genuine story of enslavement, both literal and emotional. As for Lolita, another story of enslavement, that book, an unassailable masterpiece, is going nowhere in the 'mummy-porn' market," he says.

Pearson says female fans of Fifty Shades "see a direct lineage from the novels of Jilly Cooper: the schoolgirls who spent too much time in the toilet with Polo are now the solicitors reading Fifty Shades on the tube".

Beckman agrees James has more in common with Jackie Collins than Henry Miller. "The fantasy is about a billionaire who sweeps you off your feet when you're struggling to find a job. When times are good people read misery memoirs. When times are bad, people read bonkbusters." Certainly, the size of Christian's bank balance seems as important as the size of the bulge in his "old, faded, ripped Levis".

These were the stereotypes North hoped to render obsolete. "It's a strange paradox - the gazillion female readers who feel they're emancipated by so brazenly reading this book. And yet they're lauding a totally dysfunctional relationship that I would sincerely hope none aspire to. Had this book been written by a man … a large proportion of female readers would be boycotting it."

But, the counter-argument goes, Fifty Shades wasn't written by a man, nor is it being read by them. Instead, James has shaped an erotic form that doesn't objectify women, but embraces them. "Women are looking for escapism, a release from their high-flying professional career, or family role," Siemaszkiewicz says.

Australian author Gemmell believes James is a trailblazer. "The new, wondrous and startling thing is that women have been freed to talk about erotic fiction they may previously only have read in secret."

Not everyone is convinced about these public displays of erotic affection. Male readers sound especially confused. "Why are people getting off on this?" Beckman wonders. "Especially people reading it on the train? Are they getting off on the book, or the fact they are reading it in public?"

Pearson is equally baffled. "I really have no idea why it's socially acceptable to be seen to be reading it by the pool. Although, given that adults were perfectly happy reading Harry Potter in public, Fifty Shades does represent some kind of advance."

How profound an advance remains to be seen. The bandwagon is certainly rolling: Bared to You by Sylvia Day, America's answer to James, is being marketed as "hotter than Fifty Shades". Gemmell's backlist has had a Fifty Shades make-over, re-published with now-familiar monochrome covers.

Siemaszkiewicz is taking advantage of erotica's surge into the mainstream by releasing "Clandestine Classics", in which some X-rated action is inserted into Jane Austen and the Bronte sisters. Here's Pride and Prejudice: "When his large manhood was revealed, it sat firm and proud against his stomach and though she had seen it before, the length and girth of it still surprised her, making her wonder again how it was possible it could fit."

Pearson raises an eyebrow at these innovations. "I don't think 'Sex and Sensibility' is going to be an improvement on the original. And who said Austen was sexless anyway?"

Can we expect an Asian E.L. James any time soon? Vittachi is unsure. "I've met both of the best-known modern Chinese erotic writers, Mian Mian and Wei Hui, and neither has had a fraction of E.L. James' success, despite having bigger potential markets and almost zero competition. They're probably steaming in their negligees, trying to work out where they went wrong."

Of course, it's just possible the next E.L. James is sitting or standing beside you at this very moment.

"There are people in Hong Kong who write erotic literature, but keep it secret," Vittachi says. "I knew one married mother-of-two who wrote stories to order for a single reader. He would pay her in expensive gifts from lingerie shops. Her husband didn't mind. It saved him money."